1. Field of the Invention
The invention relates generally drill bits used to drill wellbores through earth formations. More specifically, the invention relates to hardfacing structures applied to drill bits and methods for applying the same so as to reduce erosion of the drill bit during drilling operations.
2. Background Art
Drill bits used to drill wellbores through earth formations generally are made within one of two broad categories of bit structures. Drill bits in the first category are generally known as “fixed cutter” or “drag” bits, which usually include a bit body formed from steel or another high strength material and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements may be formed from any one or combination of hard or superhard materials, including, for example, natural or synthetic diamond, boron nitride, and tungsten carbide.
Drill bits of the second category are typically referred to as “roller cone” bits, which usually include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is typically formed from steel or another high strength material. The roller cones are also typically formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as is the cone. These bits are typically referred to as “milled tooth” bits. Other roller cone bits include “insert” cutting elements that are press (interference) fit into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.
Application of hardfacing to the base material from which the cones and drill bit are formed is known in the art. The hardfacing can be applied in the form of special erosion protection inserts used in addition to the cutting elements. See for example, U.S. Pat. No. 3,952,815 issued to Dysart. Another method known in the art that uses hardfacing to protect roller cones is described in U.S. Pat. No. 5,291,807 issued to Dysart. The method in the Dysart '807 patent includes marking the face of a roller cone by masking or etching, applying hardfacing material, such as tungsten carbide, in the form of a powder, and heating the cone to bond the hardfacing powder to the cone. U.S. Pat. Nos. 3,461,983 and 3,513,728 issued to Hudson include disclosure related to drilling holes (sockets) in the cone prior to application of the hardfacing, plugging the holes, and then applying the hardfacing material using a flame application process. After applying the hardfacing material with the flame process, the plugs are removed and the inserts are pressed into the previously drilled sockets.
Moreover, U.S. Pat. No. 5,348,770 issued to Sievers discloses a method for applying hardfacing to a cone which uses a high velocity oxygen fuel (HVOF) spray process after the cone is formed. Forming the cone includes drilling the sockets for the inserts. U.S. Pat. No. 4,396,077 issued to Radtke discloses a method for applying hardfacing to a fixed cutter bit. The method includes generating an electric arc and spraying arc-heated hardfacing material onto a substantially completely assembled bit structure.